Track number of RefCounted object instances.

This patch introduces InstanceCounter class to keep map<classname, counter>. The counter is incremented/decremented from RefCounted c-tor/d-tor. As RTTI is not available in Blink, we use output from __PRETTY_FUNCTION__ macro to stringify typename from a template parameter T.

Internals::dumpRefCountedInstanceCounts() API is provided to get results. This is to be used from the Blink Leak Detector.

with patch: binary size: 886M out/Debug/lib/libwebkit.so
            time run_webkit_tests --debug: 721.32s user 284.36s system 156% cpu 10:44.45 total

without patch: binary size: 882M out/Debug/lib/libwebkit.so
            time run_webkit_tests --debug: 710.66s user 279.06s system 182% cpu 9:01.73 total

Review URL: https://codereview.chromium.org/94343002

git-svn-id: svn://svn.chromium.org/blink/trunk@163063 bbb929c8-8fbe-4397-9dbb-9b2b20218538